doc/go1.16: document switch to MADV_DONTNEED

Updates #42330.

Change-Id: Ifda10a5c3dca30acf1258e9e0af202e9beffc68e
Reviewed-on: https://go-review.googlesource.com/c/go/+/267137
Trust: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

doc/go1.16.html

@@ -161,6 +161,18 @@ <h2 id="runtime">Runtime</h2>
   TODO
 </p>
 
+<p><!-- CL 267100 -->
+  On Linux, the runtime now defaults to releasing memory to the
+  operating system promptly (using <code>MADV_DONTNEED</code>), rather
+  than lazily when the operating system is under memory pressure
+  (using <code>MADV_FREE</code>). This means process-level memory
+  statistics like RSS will more accurately reflect the amount of
+  physical memory being used by Go processes. Systems that are
+  currently using <code>GODEBUG=madvdontneed=1</code> to improve
+  memory monitoring behavior no longer need to set this environment
+  variable.
+</p>
+
 <h2 id="compiler">Compiler</h2>
 
 <p>